The present invention is related to the field of interference suppression in wideband communications systems such as spread-spectrum communications systems.
The explosive growth of wireless communications has necessitated new and innovative approaches to assigning and using the fundamentally limited frequency spectrum. One proposed solution is to permit a given spectral band to be shared by two or more user communities that employ different signaling methods, provided the signals produced by one group of users don't materially affect the communications efficacy of the others. One practical method is to allow wideband, spread-spectrum communications to be conducted in the same frequency bands that support narrowband users. In this approach a wideband transmitter spreads its energy over a much larger portion of the allocated band than do the narrowband transmitters. Because a narrowband receiver is sensitive to narrowband signals, it intercepts only a small fraction of the energy transmitted by a wideband user. The effect of this small amount of interference on the narrowband system is commensurately small and generally negligible.
A similar argument does not apply for a spread spectrum user. Because it is sensitive over a wide band, a wideband receiver intercepts all the narrowband signals in its band in addition to the wideband signal of interest. Each of the interfering narrowband signals is received at full energy. Such interfering signals can significantly degrade communications performance by overwhelming the receiver with strong interfering energy and/or by causing transmitter power control algorithms to compensate for the interference by increasing the transmit power level. The latter can have the effect of increasing the level of interference caused to narrowband users by the spread spectrum system, thereby degrading the quality of service for narrowband as well as wideband users.
Frequency domain excision techniques have been used to address the problem of narrowband interference in wideband systems. In frequency domain excision, a Fourier transform is applied to a sampled version of the received baseband communications signal to convert the input time waveform into the frequency domain. The Fourier transform is typically implemented in digital form using the Fast Fourier Transform (FFT) algorithm. Once the frequency domain representation has been generated, the locations of the interfering signals are determined, generally by identifying anomalous peaks in the frequency-domain spectrum. One or another type of non-linear processing is then performed on the spectral coefficients in order to suppress the effects of unwanted narrowband signals. The modified frequency domain coefficients are then transformed back to the time domain using an inverse FFT in order to construct the output signal.
One limitation of such conventional frequency-domain processing is that a narrowband interfering signal generally appears in more than one FFT coefficient, or frequency bin, even though the actual frequency of the interfering signal may be localized to a single bin. This problem arises due to the poor frequency sidelobe structure of the FFT. The excision system eliminates many more FFT coefficients than necessary, resulting in serious degradation of receiver performance despite the removal of the interfering signal.
The usual solution to the frequency sidelobe problem is to apply a multiplicative window to each input block of samples prior to computing the Fourier transform. Specifically, if the N-point window function is denoted W(n) and the input data for the kth block is denoted as X(k,n), then windowed data Xw(k,n) which is used as the input to the Fourier transform is given by Xw(k,n)=X(k,n)×W(n). Several popular window functions include the Bartlett, Blackman, Chebyshev, Hamming, Hanning and Kaiser windows. All of these have the same general shape, in which they are symmetric about their mid-point and monotonically decrease from their largest value at the center, to zero or near-zero at the end points. Depending on which window is used, varying amounts of sidelobe suppression can be achieved in exchange for somewhat reduced frequency resolution.
Although the application of an input window reduces or eliminates the sidelobe problem and facilitates removal of only those frequency bins that truly contain interfering signals, it also introduces distortion into the reconstructed time sequence produced by the inverse Fourier transform. This distortion affects the performance of the downstream receiver demodulator. Several techniques have been suggested for mitigating window-induced distortion, such as the use of adaptive, time-varying demodulation techniques and the use of transforms other than the Fourier transform. These techniques suffer from computational complexity and relatively high cost.
One key aspect of excision performance is the estimation of appropriate thresholds that are used to distinguish desirable signals from interfering signals. Currently, various sliding window averages and median filtering techniques are used. In addition to being computationally costly, the performance of many of these methods degrades significantly in the presence of large numbers of interfering signals. Additionally, many excision techniques require the use of specialized demodulation logic in the receiver, resulting in added cost, complexity and power consumption.